A wheel-speed sensor, for example, is used for detecting the rotational speed of the wheels of a motor vehicle. A conventional wheel-speed sensor is then described. As shown in FIG. 5, a wheel-speed sensor 110 has a detection unit 107 inserted from an opening 101a of a capped tubular casing 101 thereinto and attached to the cap. A resin holder 102 is fit in opening 101a. 
A hot melt 103 is applied entirely between the edge of opening 101aand holder 102 to close the space therebetween. Then, opening 101aand an exposed part of holder 102 are covered with a resin portion 104.
Detection unit 107 of wheel-speed sensor 110 is disclosed, for example, in Japanese Patent Laying-Open Nos. 5-302932 and 7-198736 that is formed of a magnetic detection IC chip and its control electronic components (circuit).
Japanese Patent Laying-Open No. 10-260194 discloses a component employed as a detection unit that has a pole piece provided inside a bobbin having a coil wounded and held thereon and a magnet behind the pole piece.
As shown in FIG. 5, wheel-speed sensor 110 is placed to face a rotating object to be detected, namely object B. A change in a magnetic field caused by rotation of object B is sensed by detection unit 107 and converted into an electric signal. Detection unit 107 is connected to an output line 106 via a relay terminal 105.
To object B, a ferromagnetic pulsar ring with gear-like tooth around its perimeter is applied. Moreover, a magnetic pulsar ring is applied that is magnetized to exhibit south poles and north poles located alternately and successively around its perimeter in the circumferential direction.
Suppose that the above-described wheel-speed sensor is used for such a large-sized vehicle as a large motortruck, for example. As the distance traveled by the large-sized vehicle is longer than that of the normal-sized car, the temperature change of the chassis of the large-sized vehicle is greater. For this reason, it is required that wheel-speed sensor 110 ensures its detection accuracy for a temperature change in the range, for example, from −40° C. to 160° C.
At a temperature of approximately 160° C., however, hot melt 103 starts to be melted (softening temperature of hot metal: 150° C. –160° C.). In particular, molten hot melt 103 entering detection unit 107 causes deterioration of internal electric wires and the like. In addition, molten hot melt 103 deteriorates sealing of wheel-speed sensor 110 and consequently impairs detection accuracy.